1. Field of the Invention
This invention relates to a noise reducing device applied to noise generated by a noise source whose level is frequently varied, more particularly to one applied to a noise source such as an engine or muffler in an automobile, fan, or the like.
2. Description of the Related Art
Mufflers, sound insulating materials, sound absorbing materials, and the like are used for reducing noise in houses, factories, vehicle cabins, and the like.
Of particular note are those used in cabins of automobiles, aircraft, trains, and the like.
FIG. 1 shows one example of a conventional noise reducing device, which includes a microphone 14, a delaying means 2 connected to the microphone, a power amplifier 18 connected to the delaying means, and a speaker 15 connected to the power amplifier. When a noise wave a is generated at a noise source point Pa, the noise wave a is converted to an electric signal by the microphone 14. The electric signal is then delayed in the delaying means 2. The power of the delayed signal is amplified at the power amplifier 18 and then converted to actual sound in the speaker 15.
In this device, therefore, sound delayed in a phase from the original noise wave a can be generated from the speaker 15 as a noise silencing wave b. When the noise wave a, generated at a noise source point Pa, reaches a monitoring point Pb, the speaker 15 emits the noise silencing wave b. As this has a reverse phase from the noise wave a at the monitoring point Pb, it erases the noise wave a to reduce the level of the noise.
This kind of noise reducing device works for noise having stable peak frequencies but there are many noise sources which generate noise having variable peak frequencies, for example, an internal combustion engine used in an automobile or a fan used in a gas exhausting duct or the like in which the fan is rotated by an alternating current. The noise reducing device as explained above does not work well for such noise. Many problems arise in dealing with such noise. There problems will be explained with reference to automobiles hereunder, but are not restricted to automobiles.
To reduce the level of noise introduced into a cabin of an automobile, the stiffness of the car body is increased or the sound silencing performance of the muffler is increased. These methods, however, sometimes increases the weight of the car body, reducing the engine performance and increasing production costs. Therefore, a more economical noise reducing means has been required.
The noise reducing device shown in FIG. 1 is economical, but when used for reducing noise in a cabin of an automobile, problems arise. Specifically, in an automobile, noise having a low frequency is usually generated. The noise, i.e., a gas exhaust sound, is generated by the pressure variation caused by the exhaust gas periodically output from the engine. The frequency usually varies in accordance with the rotation number of the engine. This is clear from FIG. 2, which shows general relationship among a speed of an automobile, a rotation number of an engine, and a frequency of noise generated from the engine. It is difficult to form a noise silencing wave b having a reverse phase from the original noise wave a when the frequency of the noise wave a introduced inside the cabin, varies in accordance with the rotation number of the engine.
Generally speaking, the sound frequency level which can be caught by the human ear, is from 50 Hz to 5 kHz. As shown in FIG. 2, the noise generated from an engine of an automobile usually has a frequency of 50 Hz to 100 Hz. Therefore, this noise should be removed to make the cabin environment more comfortable.
On the other hand, when the noise generated from an engine is analyzed, it can be understood that the noise has a peak level at a frequency of around 100 Hz with several higher harmonic waves, as shown in FIGS. 3 and 4.
FIG. 3 shows a distribution of noise levels of noise generated from an engine at 2800 rpm, while FIG. 4 shows the same at 3500 rpm. As apparent from these figures, the peak level appears at the frequency of 100.0 Hz at 2800 rpm and of 125.0 Hz at 3500 rpm. This shows that the peak level of the noise varies depending upon the rotation number of the engine. It is most important that this peak level be reduced or erased. As far as the other higher harmonic waves are concerned, they are not so high in level, so do not usually cause strict problems to passengers.